The present invention is related to multimedia networks, and in particular to transporting a compressed media data stream, e.g., an MPEG video stream that includes null packets on a packet network by removing the null packets while maintaining the timing information.
Video on Demand (VoD) and cable operators typically store and send compressed media streams such as MPEG2 and MPEG4 streams to an endpoint such as a subscriber as constant bit rate (CBR) streams. Hybrid fiber-coax networks (HFC networks) are commonly used for transporting the compressed media streams and typically work best when the media stream is a substantially constant bit rate (CBR) stream at a bit rate that matches the available bandwidth of the HFC link. In that manner, any buffer at an endpoint is then not likely to overflow and/or underflow.
HFC networks combine both optical-fiber and coaxial cable lines. As an example, an optical fiber runs from a cable head end to feeders that feed neighborhoods of 500 to 2,000 subscribers, and then a coaxial cable runs from the optical-fiber feeder to each subscriber. The subscriber typically includes a set top box that decodes and plays back the media stream. Hybrid networks provide many of fiber's reliability and bandwidth benefits at a lower cost than a pure fiber network.
Many media streams, e.g., the streams of a typical video program received via satellite or the stream of a video program stored in a DVD intrinsically have a variable bit rate (VBR). Such a VBR stream is converted to a CBR stream, for example for storage in VoD servers or cable head ends for sending to subscribers. One common method for this conversion is to insert additional packets that do not carry significant information—called null packets herein—at appropriate locations in the stream to make the streams substantially CBR for the available bandwidth. The insertion is carried out in a manner that retains the timing information of the non-null packets, e.g., the video, audio, PSI, etc., packets in the case of MPEG2 streams. When an endpoint such as a set top box decodes and plays back the media stream, the null packet are essentially discarded.
Thus, it is known to convert a VBR media stream of a program to a substantially CBR stream by inserting null packets.
There recently has been a move to transport compressed media streams on IP networks. Such media streams are transported using UDP or RTP over UDP. The IETF RFC 2250 titled “RTP Payload Format for MPEG1/MPEG2 Video,” for example, describes a packetization scheme to transport MPEG video using RTP over UDP, i.e., using IP/UDP/RTP packets. The IETF is presently drafting similar schemes for transporting MPEG4 streams. Other methods of encapsulating compressed media streams may also be or become known, although RTP is assumed herein.
When MPEG is transported over IP, there typically is an MPEG to IP gateway on the sending side to encapsulate the MPEG stream and an IP to MPEG gateway on the receiving side to carry out such processing as jitter removal. Often, the IP to MPEG gateway also multiplexes the packets of several MPEG streams of single programs into an MPEG stream of multiple programs.
In the case that the MPEG stream of a single program is a CBR stream, the IP to MPEG gateway also rebuilds the individual CBR MPEG streams prior to multiplexing. The timing of the information-carrying media packets needs to be retained or recovered. Null packets can be locally added as needed.
When an MPEG stream that has been made substantially CBR by adding null packets is to be transported over IP, a typical prior art MPEG to IP gateway encapsulates the CBR MPEG streams, including the null packets used for padding, into IP/UDP/RTP packets and send them on via the IP network. This method is wasteful of bandwidth because the null packets contain bits that do not contain much useful information; their purpose is to make the encapsulated MPEG streams substantially CBR and to retain the timing for the media packets.
There thus is a need for a method and apparatus to encapsulate an MPEG stream that may include null packets to IP packets without encapsulating all the null packets. The method should provide for reconstructing the substantially CBR stream from the IP packets. There further is a need for a method and apparatus that sends the IP packets containing the encapsulated MPEG stream. There further is a need for a method and an apparatus that receives the IP packets encapsulating the MPEG stream packets and reconstructing the timing of the encapsulated MPEG packets, e.g., to reconstruct an essentially CBR MPEG stream. The stream reconstructing may include adding null packets as necessary, while maintaining the timing information of the packets.
Removing the null packets at the MPEG to IP gateway before sending the streams on the IP networks provides savings on bandwidth.